Re: early 70's Merc 7.5
anyone
I have no experience with a spun prop other to know that you can mark it, run it and then look at the mark to see if it's slipping around the hub. Search on that subject to find details.
First you have to figure out what the real problem is - poor running or spun prop. If it's poor running, you need to methodically check out every possible cause.
We still need to know when the last service was and what was done. Regardless though, if it's just not revving out and putting out the power and speed it should, there's no alternative to doing a complete top to bottom assessment:
Compression: a basic step. Post your numbers when you get them.
Fuel: fuel itself, tank condition, venting, pickup, all lines, connections, primer bulb, fuel pump, carb. Could also have to check the reeds, but that's more advanced (never done it).
Electrical. You need to assess spark first by getting the engine to warm idle in water or in tank and pulling one plug wire and observing what happens - whether it indicates one or the other plug is not firing.
- pulled wire has no effect on idle: that cylinder is not firing properly, other is
- pulled wire causes engine to kill - that cylinder is firing properly, other one is not.
- pulled wire causes reduced rpms, rougher idle: both cylinders are firing
You need to check all wires for condition and connection.
Spark strength using a adjustable gap tester. Both plugs could be firing intermittently or weakly.
Coils vs. switch box vs. stator. If it has a weak or non-existent spark on one cylinder, does it follow the coil or not.
Thers's some other things, but these are basic, fairly simple things I personally have experience with.
Again, I'm still not clear on what's happening. Do you think it's revving up normally or weakly? I can't tell if you don't think it is, or if you think it is and the prop isn't going well. If it's not revving up as you'd expect, it's not "running great", which is why I'm confused.
